fix(mount): do not truncate shares not zfs mount

When running zfs share -a resetting the exports.d/zfs.exports makes
sense the get a clean state.
Truncating was also called with zfs mount which would not populate the
file again.
Add test to verify shares persist after mount -a.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Stefan Lendl <s.lendl@proxmox.com>
Closes #15607 
Closes #15660

cmd/zfs/zfs_main.c

@@ -7230,7 +7230,8 @@ share_mount(int op, int argc, char **argv)
 		pthread_mutex_init(&share_mount_state.sm_lock, NULL);
 
 		/* For a 'zfs share -a' operation start with a clean slate. */
-		zfs_truncate_shares(NULL);
+		if (op == OP_SHARE)
+			zfs_truncate_shares(NULL);
 
 		/*
 		 * libshare isn't mt-safe, so only do the operation in parallel

tests/runfiles/common.run

@@ -318,7 +318,8 @@ tags = ['functional', 'cli_root', 'zfs_set']
 [tests/functional/cli_root/zfs_share]
 tests = ['zfs_share_001_pos', 'zfs_share_002_pos', 'zfs_share_003_pos',
     'zfs_share_004_pos', 'zfs_share_006_pos', 'zfs_share_008_neg',
-    'zfs_share_010_neg', 'zfs_share_011_pos', 'zfs_share_concurrent_shares']
+    'zfs_share_010_neg', 'zfs_share_011_pos', 'zfs_share_concurrent_shares',
+    'zfs_share_after_mount']
 tags = ['functional', 'cli_root', 'zfs_share']
 
 [tests/functional/cli_root/zfs_snapshot]

tests/zfs-tests/tests/Makefile.am

@@ -914,6 +914,7 @@ nobase_dist_datadir_zfs_tests_tests_SCRIPTS += \
 	functional/cli_root/zfs_share/zfs_share_012_pos.ksh \
 	functional/cli_root/zfs_share/zfs_share_013_pos.ksh \
 	functional/cli_root/zfs_share/zfs_share_concurrent_shares.ksh \
+	functional/cli_root/zfs_share/zfs_share_after_mount.ksh \
 	functional/cli_root/zfs_snapshot/cleanup.ksh \
 	functional/cli_root/zfs_snapshot/setup.ksh \
 	functional/cli_root/zfs_snapshot/zfs_snapshot_001_neg.ksh \

tests/zfs-tests/tests/functional/cli_root/zfs_share/zfs_share_after_mount.ksh

@@ -0,0 +1,62 @@
+#!/bin/ksh -p
+#
+# CDDL HEADER START
+#
+# The contents of this file are subject to the terms of the
+# Common Development and Distribution License (the "License").
+# You may not use this file except in compliance with the License.
+#
+# You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
+# or https://opensource.org/licenses/CDDL-1.0.
+# See the License for the specific language governing permissions
+# and limitations under the License.
+#
+# When distributing Covered Code, include this CDDL HEADER in each
+# file and include the License file at usr/src/OPENSOLARIS.LICENSE.
+# If applicable, add the following below this CDDL HEADER, with the
+# fields enclosed by brackets "[]" replaced with your own identifying
+# information: Portions Copyright [yyyy] [name of copyright owner]
+#
+# CDDL HEADER END
+#
+
+#
+# Copyright (c) 2023 by Proxmox. All rights reserved.
+#
+
+. $STF_SUITE/include/libtest.shlib
+
+# DESCRIPTION:
+# Verify that nfs shares persist after zfs mount -a
+#
+# STRATEGY:
+# 1. Verify that the filesystem is not shared.
+# 2. Enable the 'sharenfs' property
+# 3. Verify filesystem is shared
+# 4. Invoke 'zfs mount -a'
+# 5. Verify filesystem is still shared
+
+verify_runnable "global"
+
+function cleanup
+{
+	log_must zfs set sharenfs=off $TESTPOOL/$TESTFS
+	is_shared $TESTPOOL/$TESTFS && \
+		log_must unshare_fs $TESTPOOL/$TESTFS
+	log_must zfs share -a
+}
+
+
+log_onexit cleanup
+
+cleanup
+
+log_must zfs set sharenfs="on" $TESTPOOL/$TESTFS
+log_must is_shared $TESTPOOL/$TESTFS
+log_must is_exported $TESTPOOL/$TESTFS
+
+log_must zfs mount -a
+log_must is_shared $TESTPOOL/$TESTFS
+log_must is_exported $TESTPOOL/$TESTFS
+
+log_pass "Verify that nfs shares persist after zfs mount -a"